A fuel cell is generally known as an electricity generating system which directly converts chemical energy into electric energy through an electrochemical reaction between oxygen, or air containing the oxygen, and hydrogen contained in hydrocarbon-grouped materials such as methanol and natural gas. Specifically, the fuel cell has a feature that it can produce electricity through the electrochemical reaction between hydrogen and oxygen without combustion and provides heat as a byproduct thereof that can be used simultaneously.
Fuel cells are classified into a phosphate fuel cell working at a temperature of about 150° C. to 200° C., a molten carbonate fuel cell working at a high temperature of about 600° C. to 700° C., a solid oxide fuel cell working at a high temperature of 1,000° C. or more, and a polymer electrolyte membrane fuel cell (PEMFC) and an alkali fuel cell working at room temperature or a temperature of 100° C. or less, depending upon the kind of electrolyte used. These fuel cells work basically on the same principle, but are different from one another in kind of fuel, operating temperature, catalyst, and electrolyte.
The recently developed polymer electrolyte membrane fuel cell (PEMFC) has an excellent output characteristic, a low operating temperature, and a fast starting and response characteristic as compared with other fuel cells, and uses hydrogen obtained by reforming methanol, ethanol, natural gas, etc. Accordingly, the PEMFC has a wide range of applications such as a mobile power source for vehicles, a distributed power source for the home or buildings, and a small-sized power source for electronic devices.
The aforementioned PEMFC has a fuel cell main body (hereinafter, referred to as a stack), a fuel tank, and a fuel pump supplying fuel to the stack from the fuel tank, to constitute a typical system. Such a fuel cell further includes a reformer for reforming the fuel to generate hydrogen gas and supplying the hydrogen gas to the stack. Therefore, in the PEMFC the fuel is stored in the fuel tank is supplied to the reformer by means of pumping power of the fuel pump. The reformer then reforms the fuel and generates the hydrogen gas. The stack makes the hydrogen gas and oxygen electrochemically react with each other, thereby generating electric energy.
Alternatively, such a fuel cell can employ a direct methanol fuel cell (DMFC) scheme directly supplying liquid fuel containing hydrogen to the stack and generating electricity. The fuel cell employing the direct methanol fuel cell scheme does not require the reformer, unlike the PEMFC.
In the fuel cell system described above, the stack substantially generating the electricity has a stacked structure of several or several tens of unit cells having a membrane-electrode assembly (MEA) and a separator (or a bipolar plate). The MEA has a structure such that an anode electrode and a cathode electrode are bonded to each other with an electrolyte membrane therebetween. The separator simultaneously performs a function of a passage through which oxygen and hydrogen gas required for the reaction of the fuel cell are supplied, and a function of a conductor connecting the anode electrode and the cathode electrode of each MEA to each other in series.
Therefore, through the separator, hydrogen gas is supplied to the anode electrode and oxygen is supplied to the cathode electrode. An oxidation reaction of the hydrogen gas takes place in the anode electrode and a reduction reaction of oxygen takes place in the cathode electrode. Due to movement of electrons generated at that time, electricity, heat, and water can be collectively obtained.
The reformer described above is an apparatus which converts through a catalytic reformation reaction the liquid fuel containing hydrogen and water into the hydrogen gas required for generation of electricity by the stack, and, in addition, which removes noxious substances such as carbon monoxide which poisons the fuel cell and shortens its lifetime. The reformer includes a reforming section for reforming the fuel and generating the hydrogen gas, and a carbon-monoxide removing section for removing carbon monoxide from the hydrogen gas. The reforming section converts the fuel into reformed gas that is rich in hydrogen through a catalytic reaction such as steam reformation, partial oxidation, natural reaction, etc. The carbon-monoxide removing section removes carbon monoxide from the reformed gas using a catalytic reaction such as a water-gas shift reaction, an oxidation reaction, etc., or hydrogen purification with a separating membrane.
In the conventional reformer of a fuel cell system, the reforming section includes an exothermic reaction portion inducing a catalytic oxidation reaction between fuel and air and generating combustion heat, and an endothermic reaction portion accepting the combustion heat, inducing a catalytic reformation reaction of the fuel, and generating the hydrogen gas. Therefore, in the reforming section, the endothermic reaction portion accepts a predetermined reaction heat from the exothermic reaction portion and generates the reformed gas having rich in hydrogen from a mixed fuel of liquid fuel and water through the catalytic reformation reaction.
However, since the conventional reformer of a fuel cell system has a structure such that the reaction heat can easily leak outside, the reforming section, which needs a uniform temperature distribution, exhibits an uneven temperature distribution, such that reaction efficiency and thermal efficiency of the overall reformer deteriorates.